In many situations, a time display is required at various locations around large installations. Often, it is also essential that the time is very accurately displayed at each location. An example of a particular application is the broadcasting industry where the programmes transmitted via a network are often produced in various studios and it is essential that the predetermined time for switching between studios is accurately displayed at all locations.
There are currently two main types of system which are commonly used to provide an accurate time display at a number of locations. The first of these consists of a number of synchronous clocks running from the mains supply and maintaining the time to the accuracy of the frequency of the mains power. In the second commonly used system, the clocks at each location are synchronised to a master clock at a central location. This arrangement necessitates having all the clocks connected via wires to the central distribution centre. Naturally either of these methods can utilize either analogue or digital displays, or a mixture of both.
Unfortunately each of these systems has a number of inherent disadvantages. In the first system, should a blackout occur all the clocks will display the incorrect time at restoration of power and it will be necessary to reset each and every clock. Similarly, at the beginning and end of daylight saving all the clocks must again be corrected by one hour.
The second system can overcome some of the disadvantages inherent in the first, but often the solution is less than satisfactory. In this respect, the "slave" clocks at each location are usually powered from the master and if auxiliary power is available the system will survive a blackout. Adjustment of the clocks at the beginning and end of daylight saving is also possible. Advancement of the clocks is usually effected by advancing the master clock at a rate which the slaves can follow perhaps taking ten minutes to advance the hour. The retardation is often less elegant. Sometimes the master clock is effectively stopped for an hour, thus restoring the correct time.
There are also further problems peculiar to analogue displays, which being mechanical devices are prone to stick, jam, or otherwise misbehave. Accordingly, many mechanical slave clocks utilize a checking mechanism which usually, requires that the hands of the clock are in a certain position at the predetermined checking time. This function is usually effected by having the master output a series of pulses indicative of correct hand position. If the hands of the clock are in the correct position, then these pulses are ignored, but if they are not in the correct position the clock is stepped forward to the correct time.
It will be appreciated that in broadcasting an analogue display incorporating a sweep hand is particularly useful in advising a disc jockey, for example, of the number of seconds remaining before a news bulletin. Digital displays cannot achieve this result as the seconds display moves too quickly to be assimilated.